In greenhouses (using daylight) and growth chambers (without daylight) plants are grown throughout the year. The closed environment of a greenhouse or growth chamber requires good control of different parameters in order to provide for optimal photosynthesis of the plants. Three of the most important parameters that control the photosynthesis and thus the plant growth are the ambient light intensity, the temperature and the concentration of carbon dioxide (CO2). There are today various control systems for monitoring and/or controlling the growth of plants by controlling one or more of said parameters, e.g. using CO2 generators for increasing the CO2 concentration in the greenhouse or growth chamber, and additional lighting arrangements.
The addition of CO2 using conventional systems is effected by raising the general concentration of CO2 in the greenhouse usually to about 1300 ppm. However, a disadvantage of such systems is that the CO2 concentration close to the leaves of the plants, where the photosynthesis takes place, might be lower (e.g. due to depletion), and not high enough to achieve the desired increase in the photosynthesis.
To provide additional lighting for greenhouses and growth chambers, high brightness, highly efficient light emitting diodes (LEDs) are becoming more and more interesting because of their low energy consumption, good efficiency, low cost, and the possibility of adapting the color output.
The light output of an LED depends on a number of factors such as the brightness of the LED, any optics used to create a certain light pattern, the current delivered to the LED, and the junction temperature of the LED (i.e. the temperature of the light emitting portion of the LED). In general the light output of an LED is reported for a particular junction temperature. The light output decreases with increasing temperature generated by the operation of the LED, and the efficiency of the LED is thus reduced for higher temperatures. Furthermore, the lifetime of the LED is also influenced by the junction temperature, higher junction temperatures decreasing the lifetime of the device.
Various methods may be used to cool LEDs during operation. One commonly used technique is to thermally couple the LED to a heat sink which dissipates the heat generated by the LED into the ambience. Alternatively, forced air or liquid may be used to cool an LED. These methods may result in a reduction in junction temperature sufficient in order to obtain the desired light output. However, the constant desire to increase the individual LED light output creates a need for thermal management that goes beyond the capabilities of a conventional heat sink.
WO 2007/093607 discloses a lighting device for stimulating the growth of plants. The lighting device has a solid state light source for emitting light of at least one wavelength within a predetermined wavelength range. The solid state light source is in contact with a cooling medium, the cooling medium having a temperature in the range between −50° C. and 0° C., preferably between −50° C. and −20° C. However, the use of such cold cooling medium is a disadvantage, requiring that the medium be encapsulated in at least two tubes.
Hence, there is clearly a need in the art for improved LED based lighting devices, and in particular lighting devices for use in large scale greenhouses and growth chambers to promote or control growth of plants.